This application is related to commonly-assigned U.S. patent application Ser. No. 12/331,824 , filed Dec. 10, 2008.
The present invention relates generally to the operation of a turbomachine, and more particularly, to a method of reducing the variation associated with the start-up time of the turbomachine.
“Fast Start” may be considered an operating mode requiring a turbomachine to export a load, capable of emissions complaint operation, within a certain time after an operator initiates a start of that turbomachine. Fluctuating energy demand is a major factor in determining when the turbomachine operates. Turbomachines are commonly idled until sufficient demand requires operation. When demand requires operation, the turbomachine performs a start-up process before exporting the desired energy (electricity, mechanical torque, steam, and the like).
The start-up process of some known turbomachines, such as, but not limiting of, a combustion gas turbine, typically involves a plurality modes occurring at different operating speeds. These modes include, but are not limited to: a purge, an ignition, a warm-up, and acceleration to an operating speed. The start-up time ranges from the initiation of a start to when the gas turbine reaches the operating speed.
During the startup process there are two sources of torque available to accelerate a rotor of the gas turbine. Energy released during the ignition process, and the torque applied to the rotor via from the starting means. The energy released during the ignition process is converted to mechanical torque, which drives the rotor. However, at lower speeds this torque is insufficient to drive the rotor to the operating speed, resulting in a negative torque, or drag, on the gas turbine. At higher speeds the mechanical torque contributed by the ignition process provide very large torque inputs. Here, the gas turbine become self-sustaining as the speed of the rotor nears the operating speed, no longer requiring torque from the starting means.
Typically, there can be wide variations in the start-up time. These variations are typically caused by a change in an operating parameter, which may require an adjustment to a fuel stroke reference (FSR) or the like, in order to accelerate the gas turbine to the operating speed. Here, FSR is adjusted because the starting means operates on a fixed schedule. Essentially, this fixed schedule provides higher torque at lower rotor speeds. The torque decreases as the rotor speed increases.
There are a few issues with the current start-up process. In some applications, variations in the start-up time may be unacceptable. Here, due to the variation in the start-up time, the gas turbine may not meet operational requirements.
Therefore, there is a desire for an improved method of starting a gas turbine. The method may reduce the variations in start-up times associated with the accelerating the gas turbine to an operating speed. The method should apply the gas turbine is operating in either: a Normal Mode, a Fast Start Mode, or the like. This method should also provide a more consistent and repeatable start-up times.